1. List of Prior Art
The prior art appears to be best exemplified by the following patents which were developed in a search:
______________________________________ Hainsworth 2,798,855 July 9, 1957 Hainsworth 2,798,856 July 9, 1957 Crone 3,078,182 Feb. 19, 1968 Carumpalos 3,288,718 Nov. 29, 1966 Edenbaum 3,360,337 Dec. 26, 1967 Bhiwandker 3,523,011 Aug. 4, 1970 Silva 3,667,916 June 6, 1972 Emigh 3,684,737 Aug. 15, 1972 Verses 3,704,096 Nov. 28, 1972 Chapman 3,862,824 Jan. 28, 1975 Banczak 4,021,252 May 3, 1977 Wachtel 4,024,096 May 17, 1977 Parkinson 4,045,397 Aug. 30, 1977 Hwang 4,070,322 Jan. 24, 1978 ______________________________________
2. Field of the Invention
The invention of this application relates to ink jet printing compositions and their use as color change indicators. The compositions are particularly useful as sterilization and/or thermal exposure indicators.
Ink jet printing techniques, although of comparatively recent development in the art of applying decorative and/or identifying indicia to a substrate, are of increasing importance. In general, such techniques impose rigid requirements on the ink compositions. To be suitable for use as a jet ink, the compositions must meet rigid requirements of viscosity and resistivity, solubility, compatibility of components and wettability of substrate; the ink must be quick-drying and smear resistant without clogging the ink jet nozzle and must permit rapid clean-up of the machine components with minimum effort. At the same time, such compositions must also be adapted for satisfactory performance in the particular end use for which they are specifically intended. For example, where the ink is to be employed in the printing of plastic or metal substrates, for example, coated and uncoated tinplate or aluminum, the ink must properly wet the surface and, where the surface is coated, it is highly desirable that some penetration of the coating be effected. In addition to proper wetting and/or penetration of the surface to be printed, the ink must also adhere strongly and be resistant to abrasion or moisture. It has been particularly difficult to obtain satisfactory adhesion and to maintain such adhesion to surfaces which are subjected to sterilization processes involving the combination of moisture and high temperature.
Apart from the jet ink applications of the ink, another area of interest in the art is the provision of printable pasteurization or sterilization indicators. Such compositions have heretofore usually utilized pigment combinations and have been applied primarily in determining whether proper sterilization heat has been applied to objects used in medical and surgical procedures. In the food packaging industry, for example, where metal cans, plastic pouches or coated metal cans are printed, filled with product, sealed and the package subjected to conditions of high temperature and moisture during pasteurization or sterilization techniques, there is a need for such compositions that are printable by either contact or jet ink techniques, that exhibit sufficient adhesion to the substrate to withstand the moisture and high temperature conditions of sterilization and which, at the same time, undergo a visible and permanent color change.
The provision of such an ink composition which shows a distinct difference in color among unsterilized and completely sterilized package provides for ready visible inspection and permits tracing a particular package or packages after they have been processed. The invention thus provides a positive and visible indication that sterilization or pasteurization has in fact been carried out on the product to which the ink is applied.
It is an object of this invention to provide ink compositions, suitable for use in ink jet printing techniques, that exhibit excellent adhesion when exposed to wet immersion or other moisture at elevated temperature conditions and which undergo a visible color change when so exposed.
It is another object of this invention to provide ink compositions, suitable for use in contact printing techniques, that exhibit excellent adhesion when exposed to moisture and elevated temperature conditions and which undergo a visible color change when so exposed.
Yet another object of this invention is to provide ink compositions capable of exhibiting excellent adhesion to both plastic or coated and uncoated metal substrates and which undergo a visible color change when exposed to moisture and high temperature.
These and other objects of the invention will be apparent from the description of the invention which follows.